This invention relates to a filter circuit and, more particularly, to a filter circuit whose cut-off frequency is easily controlled and, moreover, which can be constructed as an integrated circuit.
It is conventional to fabricate filter circuits as integrated circuits. For cost effectiveness in manufacturing such integrated circuit filters, it is important that the number of external connections which are needed for proper operation thereof, that is, the number of connecting terminals which must be provided, be minimized. Heretofore, active filters have been manufactured as integrated circuits. However, in typical IC active filters, the resistance values of the various resistive elements often cannot be as high as desired. This results in a non-uniform cut-off frequency. That is, in a particular run or batch of IC active filters, the cut-off frequency of one may differ from that of the other.
Another difficulty found in IC active filters is that, since the temperature characteristic of the resistive elements often is less than satisfactory, the cut-off frequency of the filter is, to a significant extent, dependent upon temperature. Thus, the operating characteristics of the IC active filter may become unstable with temperature deviations.
Yet another disadvantage in typical IC active filters is that a low cut-off frequency for either a low-pass filter or a high-pass filter is not easily obtainable because the resistance and capacitance values of the resistive and capacitive elements therein are not as high as desired. That is, limitations on the resistive and capacitive values prevent the filter from having a relatively low cut-off frequency.